Fibronectin is a glycoprotein that occur naturally in various contexts within animal organisms. Fibronectin occurs as a soluble protein in blood and certain other body fluids and may be isolated from blood plasma. In fact, a form of fibriconectin found in human blood plasma is a major by-product of the isolation of blood coagulation Factor VIII, also called antihemophilic factor. Antihemophilic factor is currently isolated from human blood plasma in considerable quantities for the treatment and control of the symptoms of hemophilia. Consequently, blood plasma fibronectin produced as a by-product is relatively plentiful. Having no particular use, it is generally discarded.
Fibronectin also occurs as an insoluble protein in tissue stroma. Fibronectin bonds to collagen, fibrin, heparin, heparan sulfate, eukaryotic cells, and gram positive bacteria. It is probably an opsonin, and it is also a substrate for blood coagulation Factor XIII.sub.a, also called plasma tranglutaminase.
Fibronectin occurs in subunits that are bound together in more than one way. In its soluble form, as found in plasma, fibronectin occurs primarily in a dimeric form. The insoluble forms of fibronectin are predominatly multimeric. While dimeric fibronectin is easily isolated from blood plasma, multimeric fibronectin is extremely difficult to isolate. Attempts have been made to isolate multimeric fibronectin from the surfaces of cells on which it is found. A solution of urea is mixed with the cell culture and, after agitation, the solution is separated from the cells. The protein content of the solution is then isolated. This process is unsatisfactory in many ways. It is very expensive. Furthermore, the resulting multimeric fibronectin ix mixed with unpredictable quantities of other materials incidentally stripped from the cell surface with the fibronectin. Consequently, multimeric cell surface fibronectin so produced is a poorly defined material that may vary in content from preparation to preparation. Because of the difficulty and relatively unsatisfactory nature of the preparation, the isolation of mutimeric fibronectin is rarely attempted commercially, and then primarily with cell cultures that may be conveniently managed. Commonly, only multimeric fibronectin isolated from chick embryo fibroblasts and from human foreskin fibroblasts is commercially available. Preparations of multimeric fibronectin are many times more expensive than preparations of dimeric, plasma fibronectin.
Multimeric fibronectin is a growth stimulant for cell cultures, including such diverse types of cells as kidney cells and nerve cells. Dimeric fibronectin isolated from blood plasma is also a growth stimulant, but multimeric fibronectin is much more potent. At present, multimeric fibronectin is not available in sufficient quantities or at a price economical enough to be employed in commercial cell culturing processes. Furthermore, the multimeric fibronectin that is available, being a poorly defined substance for the reasons noted above, is unsuitable even for experimental uses in situations in which reproducibility of processes is vital or in which it is necessary to be able precisely to define the contents of reagents used.